A predominant number of color picture tubes in use today have line screens and shadow masks that include slit-shaped apertures. The apertures are aligned in columns, and the adjacent apertures in each column are separated from each other by webs or tie bars in the mask. Such tie bars are essential in the mask to maintain its integrity when it is formed into a dome-shaped contour which somewhat parallels the contour of the interior of a viewing faceplate of a tube. Tie bars in one column are offset in the longitudinal direction of the column (vertical direction)from the tie bars in the immediately adjacent columns. When electron beams strike the shadow mask, the tie bars block portions of the beams, thus causing shadows on the screen immediately behind the tie bars.
When the electron beams are repeatedly scanned in a direction perpendicular to the aperture columns (horizontal direction), this scanning results in a series of bright and dark horizontal lines on the screen. These bright and dark horizontal lines interact with the shadows formed by the tie bars, creating lighter and darker areas and producing a wavy pattern on the screen, called a moire pattern. Such moire pattern greatly impairs the visible quality of image displayed on the screen. It is highly desirable to select a moire mode that will minimize the moire pattern for any scan condition used in a television receiver. The two scan conditions presently in use are interlaced scan and noninterlaced scan. A moire mode is the ratio of scan line pitch to tie bar shadow pitch. Because of the practical limitations of light output and mask strength, the moire mode is usually chosen to be between 6/8 and 10/8. The moire mode most frequently selected is 7/8. Such mode can be expressed by the equation: ##EQU1## where T.sub.s is the pitch or period of the scanning lines, which is equal to the vertical height, H, of the viewing screen divided by the number, n.sub.e, of effective scanning lines for a given TV system: and a.sub.v is the vertical repeat distance of the mask apertures on the screen.
There is a possibility for use of a third scan condition. This third condition is called progressive scan and may be used on high definition television receivers. A higher scan frequency is necessary for progressive scan. In the special case of progressive scan, only one scan condition is considered to minimize the moire pattern. This scan condition produces less moire and a much smoother picture. For this condition, a moire mode lower than 6/8 or higher than 10/8 would be used. The moire mode most frequently selected for this condition is 5/8.
There have been many techniques suggested to reduce the moire problem. Most of these techniques involve rearranging the locations of the tie bars in a mask to reduce the possibility of the electron beam scan lines beating with the tie bar shadows. Although many of these techniques have been used successfully in the past to reduce moire, most of the prior techniques do not correct the moire, problem in all parts of a screen, so that there is still a need for improved moire reduction techniques. Such improved techniques are especially needed for the newer higher quality color picture tubes that are required for higher definition television. For example, as the quality of electron guns improves to meet the needs of higher definition television, such improved guns produce smaller electron beam spots at the screen. This reduction in electron beam spot size produces visually sharper scan lines on the screen which interact with the tie bar shadows and increase the moire pattern problem.